An organic luminous element is hopeful for a display device of the next generation, because the organic luminous element is a self-luminous element, and therefore, for example, the device does not require a backlight required in a liquid crystal display device, and has a wide viewing angle.
A sectional view of an element structure of a general organic luminous element is shown in FIG. 4. The organic luminous element has a structure in which an organic layer 42 is sandwiched by a cathode 41 and an anode 43. When a DC power supply 44 is connected to this organic luminous element, holes and electrons are injected into the organic layer 42 from the anode 43 and the cathode 41, respectively. The injected holes and electrons move to the counter electrodes in the organic layer 42 due to an electric field formed by the power supply 44. The electrons and the holes are combined again within the organic layer 42 in the course of the movement to generate excitons. Luminescence is observed in a process in which energy of the excitons is deactivated. Luminescent colors are different depending upon energy inherent in the excitons, and light having a wavelength of energy substantially corresponding to a value of an energy band gap inherent in the organic layer 42 is generated.
In order to take out the light generated in the organic layer to the outside, a material, which is transparent in a visible light region, is used for at least one of the electrodes. A material, which has a low work function, is used for the cathode in order to facilitate injection of electrons into the organic layer. For example, a material such as aluminum, magnesium, or calcium is used. A material such as an alloy of these metals or aluminum-lithium alloy may be used for durability and a lower work function.
On the other hand, a material having a large ionization potential is used for the anode owing to its easiness to inject holes. In addition, since the cathode does not have transparency, a transparent material is often used for this electrode. Therefore, in general, an ITO (Indium Tin Oxide), gold, indium zinc oxide (IZO), or the like is used.
In recent years, in an organic luminous element using a low molecular material, in order to improve luminous efficiency, the organic layer 42 may be constituted by plural layers. This makes it possible to share functions of carrier injection, carrier movement to a luminous field, and emission of light having a predetermined wavelength in the respective layers. Furthermore, it becomes possible to create an organic luminous element with higher efficiency by using materials with high efficiency for the respective layers.
Luminance of the organic luminous element formed in this way is proportional to a current as shown in FIG. 5(a) and is in a nonlinear relation with respect to a voltage as shown in FIG. 5(b). Therefore, in order to perform gradation control, it is better to control the organic luminous element according to a value of current.
In the case of an active matrix type, display devices are distributed into those of two systems, namely, a voltage drive system and a current drive system.
The voltage drive system is a method of using a source driver of a voltage output type, converting a voltage into a current inside a pixel, and supplying the converted current to organic luminous elements.
In this method, since voltage/current conversion is performed by a transistor provided for each pixel, there is a problem in that fluctuation in output currents occurs and unevenness in luminance is caused according to fluctuation in characteristics of this transistor.
The current drive system is a method of using a source driver of a current output type, giving only a function of retaining a value of current, which is outputted for one horizontal scanning period, inside a pixel, and supplying the same value of current as the source driver to organic luminous elements.
Examples of the current drive system are shown in FIG. 6. The system of FIG. 6 uses a current copier system in a pixel circuit.
A circuit at the time of operation of a pixel 67 in FIG. 6 is shown in FIG. 7.
When a pixel is selected, as shown in FIG. 7(a), a signal is outputted from a gate driver 35 such that a gate signal line 61a of a row of the pixel brings a switch into a conduction state and a gate signal line 61b of the line brings a switch into a non-conduction state. A state of the pixel circuit at this point is shown in FIG. 7(a). At this point, a current flowing to the source signal line 60, which is a current attracted into a source driver 36, flows through a path indicated by dotted line 71. Thus, a current identical with the current flowing to the source signal line 60 flows to a transistor 62. Then, a potential of a node 72 changes to a potential corresponding to a current/voltage characteristic of the transistor 62.
Next, when the pixel changes to an unselected state, the circuit is changed to a circuit as shown in FIG. 7(b) by the gate signal lines 61. A current flows from the EL power supply line 64 to the organic luminous element 63 through a path of dotted line indicated by 73. This current depends upon the potential of the node 72 and the current/voltage characteristic of the transistor 62.
In FIGS. 7(a) and (b), the potential of the node 72 does not change. Therefore, a drain current flowing to the identical transistor 62 is identical in FIGS. 7(a) and (b) Consequently, a current of the same value as the value of current flowing to the source signal line 60 flows to the organic luminous element 63. Even if there is fluctuation in the current/voltage characteristic of the transistor 62, values of currents 71 and 73 are not affected in principle, and uniform display without influence of fluctuation in characteristics of a transistor can be realized.
Therefore, it is necessary to use the current drive system in order to obtain uniform display. For that purpose, the source driver 36 must be a driver IC of a current output type.
An example of an output stage of a current driver IC, which outputs a value of current according to a gradation, is shown in FIG. 10. An analog current is outputted to display gradation data 54 from the current output 104 by a digital/analog conversion unit 106. The analog/digital conversion unit is constituted by plural (at least the number of bits of the gradation data 54) current sources for gradation display 103 and switches 108, and a common gate line 107 which regulates a value of current flown by one current source for gradation display 103.
In FIG. 10, an analog current is outputted in response to the input 105 of three bits. It is selected by the switches 108 whether the current sources 103 of the number corresponding to a weight of bits are connected to the current output 104, whereby a current corresponding to a gradation can be outputted in such a manner that a current equivalent to one current source 103 is outputted in the case of data 1 and a current equivalent to seven current sources 103 is outputted in the case of data 7. A current output type driver can be realized by arranging digital/analog conversion units 106 of this structure by the number corresponding to the number of outputs of the driver. In order to compensate for a temperature characteristic of the transistors 102, a voltage of the common gate line 107 is determined by a distributing mirror transistor 102. The transistor 102 and the current source group 103 are formed in a current mirror structure, and a current per one gradation is determined according to a value of a reference current 89. With this structure, an output current changes according to a gradation, and a current per one gradation is determined according to a reference current.
Examples of a display device using an organic luminous element are shown in FIGS. 21 to 23. FIG. 21 shows a television, wherein FIG. 21(a) is a perspective view of the television and FIG. 21(b) is a block diagram showing a structure of the television, FIG. 22 shows a digital camera or a digital video camera, and FIG. 23 shows a personal digital assistant. Since a response speed of the organic luminous element is high, the organic luminance element is a display panel suitable for these display devices which has many opportunities to display motion images (For example, see Japanese Patent Application Laid-open No. 2001-147659).
In a current driver as shown in FIG. 10, transistors 103 of an identical size are arranged in a number equivalent to “number of gradations−1) and the number of transistors 103 connected to outputs is changed with respect to input data, whereby current output is performed. Therefore, a gradation and an output current are in a proportional relation. When this current is outputted directly, an image looks whitish generally due to human visual characteristics. (A low gradation side is whitish.)
In a drive device for a general display, an output according to each gradation is subjected to gamma correction. In the case of a liquid crystal display, since the liquid crystal display is voltage drive, a voltage value corresponding to each gradation is required. (In the case of a voltage, since it is impossible to represent a gradation according to addition of gradation components unlike the case of a current, a voltage is required for each gradation.) Therefore, a voltage is adjusted to a voltage value so as to be a voltage output corresponding to gamma correction and outputted at a stage of each gradation voltage. Thus, even a drive is a six-bit driver, a voltage is subjected to gamma correction, and gradation display is possible sufficiently.
On the other hand, in a current driver, since an output is not subjected to gamma correction even in the same six bits, a gradation output finer than six bits is required in order to make a pitch fine in a low gradation part. If it is attempted to achieve this with frame curtailing (FRC), frame curtailing among at least four frames is required. Moreover, since response speed of an organic luminous element is high, flicker occurs. Therefore, it is necessary to perform gradation representation without the FRC, for example, it is necessary to change an output to eight bits.
This problem is a problem peculiar to the case in which a current driver, in which a gradation and an output current are proportional, and a current output type display element, in which an input current and a luminance are proportional, are combined.
In order to eliminate gamma correction according to the FRC, it is conceivable to adopt a constitution in which an output of a current driver is increased to six bits to eight bits, gamma processing is performed before a signal is inputted to a source driver, and an eight-bit signal subjected to the gamma processing is inputted to the source driver.
As a method of expanding an output of the current driver from six bits to eight bits, there is a method of preparing 255 transistors 103. In the case of this method, four times as many transistors 103 are required compared with a conventional method (sixty-three transistors 103), and an area of the source driver also increases accordingly. Since a ratio of an area occupied by an output stage transistor in a total chip area is about seventy percent, simply speaking, the area is about three times as large as the area at the time of six bits. This imposes a significant impact in terms of cost.
In addition, in recent years, multicolor has been advanced in a portable information terminal as well, and 65,000 color or 220,000 color display has become a mainstream. In the case of an RGB digital interface, an input signal of a driver IC requires sixteen bits or eighteen bits. Therefore, sixteen to eighteen input signal lines are required only for transfer of data. Other than the input signal lines, signal lines are required for a signal for operation of a shift register, setting of various registers, and the like.
Therefore, the number of wirings increases. For example, as shown in FIG. 3, wirings between a control IC 31 and source driver ICs 36 increases with respect to a display panel 33. Consequently, there is a problem in that cost increases because, for example, a size of a flexible plate 32 increases and a multilayer substrate is used.